1. Field of the Invention
The present invention relates to an apparatus and method for controlling foam, particularly water-based foam.
2. Related Art
Industrial production processes often result in the generation of significant volumes of foam which obstructs the production process or subsequent transportation and storage of a produced material. Various proposals have been made for reducing the volume of foam generated or destroying foam after it has been generated. It is known for example to add chemicals to a foam so as to cause it to collapse. In some industrial processes such an approach is effective but often it is inappropriate to add chemicals to a foam, for example when that foam is produced as part of the production process of a food, drug or beverage. Mechanical devices for destroying foam have also been proposed, for example cyclone foam breakers. In addition it has been proposed to expose a foam to an ultrasonic beam and irradiate a foam with radiation.
In one known foam irradiation proposal, described in U.S. Pat. No. 1095934, foam which is generated during micro-organism synthesis is destroyed by exposing the foam to laser radiation at a wavelength corresponding to the spectrum of absorption of a film of the liquid contained in the foam. In another proposal described in Finnish patent specification no. FI 77788, reference is made to destroying foam by using a laser beam to cause xe2x80x9clocal heating and rupturexe2x80x9d of the bubbles making up the foam. A carbon dioxide laser source is used having an operating wavelength of 10600 nm, at which wavelength water absorption is relatively high. In a further proposal described in Japanese patent specification no. JP 63 104620, a carbon dioxide laser is used to destroy foam on beer, foam destruction in a matter of seconds being claimed.
All the above proposals were first made in the 1980""s but do not appear to have resulted in any practical devices based on the irradiation of foam with a laser. It is probable that this failure to exploit the initial proposals results from an understanding, that foam destruction with lasers relied upon absorption effects that result in heating of the foam, thus requiring relatively high power inputs and risking chemical modification of the foam.
Lasers based on the Erbium ion are known to operate at around 3000 nm which corresponds to a peak in the energy absorption spectrum for water also centred at approximately 3000 nm. For example, the Erbium:YAG laser operates at 2940 nm, and the Erbium:YSGG laser at 2970 nm. Absorption at around 3000 nm is approximately five times greater than at the carbon dioxide laser wavelength of 10600 nm, but nevertheless if the mechanism underlying the destruction of foam by lasers operating at around 3000 nm and 10600 nm was the same, using an Er based laser would not be expected to produce a dramatic improvement in performance as compared with that to be expected with a carbon dioxide laser.
It is an object of the present invention to provide an improved laser radiation foam control apparatus and method.
According to the present invention, there is provided a foam control apparatus comprising means for directing a laser beam into a vessel adapted to receive a body of foam to be controlled, wherein the laser has a wavelength selected to correspond to a mode specific peak of the absorption spectrum of a liquid constituent of the foam. Preferably, the laser generates a pulsed output and the pulse length is selected to match the thermal and mechanical properties of the foam such that energy is transferred to the foam sufficiently rapidly to disrupt the molecular structure of the foam.
The invention is based on a consideration of mechanisms resulting in foam destruction in the case of water-based foam, that is foam in which bubbles are formed by films which are primarily of water. In the water molecule, the Hxe2x80x94Oxe2x80x94H structure is bonded such that the angle between the two Oxe2x80x94H bonds is approximately 105xc2x0. Different modes of vibration exist for the molecule. In the gas phase the lowest energy vibration is a symmetric bending mode with a vibrational energy spacing of 0.1915 eV (6474 nm) and this probably causes the peak in the liquid water absorption spectrum around 6000 nm, Two stretching modes also exist, one symmetric and one asymmetric at energies of 0.4527 eV(2739 mn) and 0.4656 eV(2663 mn) in the gas phase respectively, and these probably result in the peak around 3000 nm in the liquid phase.
It is believed that an Erbium-based laser such as the Er:YAG laser pumps the asymmetric stretch mode of the molecule. It is probably the case that carbon dioxide laser absorption proceeds through rotational transitions that essentially heat the water molecules and hence the liquid. Thus, it is believed that exciting the asymmetric stretch mode of a water molecule with for example Er:YAG laser radiation injects an energy per photon that is about seventeen times larger than thermal energies and causes the Oxe2x80x94H bonds to vibrate. Each water molecule can form hydrogen bonds with up to four other water molecules in a tetrahedral structure. Thus this vibrational motion created in the laser excited molecule very effectively disturbs the (two) hydrogen bonds between the H atoms in the excited molecule and the adjacent molecules (where at the comers of the adjacent tetrahedrons there is a high probability of electron density). Thus excitation of Oxe2x80x94H bonds in one molecule causes breakage of the hydrogen bonds with nearby molecules. During laser irradiation, any molecules in a spatially localised region are excited, causing many hydrogen bonds to break, and so damaging the structural integrity of the liquid film of the foam bubble. A pulse of duration 0.25 ms for example can have a relatively low beam energy and yet deliver energy to the bubble at a rate which disrupts the molecular structure rather than simply heating the bubble wall. The destabilisation caused is amplified by the internal pressure of the bubble, causing the bubble to rupture before the hydrogen bonds can reform. In contrast, excitation by a carbon dioxide laser would cause an injection of energy per photon that is approximately five times greater than thermal (approximately one third less than for an Erbium-based laser) but the injection is not mode specific and causes bond breakage in a thermal manner. Thus although the absorption coefficient is only five times larger, the actual bubble bursting capability for 2940 nm radiation or other wavelengths close to 3000 nm is vastly superior to 10600 nm radiation because it is mode specific and leads to rapid structural collapse of the film. This very effectively bursts foam bubbles with an efficiency that is substantially greater than the factor five indicated by the relative absorption coefficients between the two laser wavelengths.
In summary, the invention is based on the realisation that the efficiency of foam destruction in comparing the a CO2 laser with a laser operating at a mode specific wavelength will not mirror the ratio of absorption coefficients because the structural impact of the two wavelengths on a thin film in tension is different because of the mode specific nature of the Erbium-based laser excitation as compared with the rotational envelope xe2x80x9cthermalxe2x80x9d heating effect of the carbon dioxide laser.
Preferably if the foam to be destroyed is water-based, the laser is an Erbium-based laser such as an Erbium:YAG laser. The laser is preferably operated with a pulse width of 0.25 ms, that is with a pulse length which matches the thermal and mechanical properties of the bubble so as to transfer energy to the bubble at a rate which optimises the bubble damaging process. Means may be provided for spatially scanning or shaping an output beam of the laser so as to maximise the volume of foam exposed to the laser beam.
The invention also provides a method for controlling foam, wherein a laser beam is directed into a vessel containing the foam to be controlled, the laser having a wavelength selected to correspond to a mode specific peak of the absorption spectrum of a liquid constituent of the foam.